U.S. Pat. No. 3,525,399 (Bayless et al.), U.S. Pat. No. 3,664,424 (Penberthy, Jr. et al.), U.S. Pat. No. 3,664,425 (Penberthy et al.), U.S. Pat. No. 3,718,184 (Bayless et al.) and U.S. Pat. No. 4,204,919 (Randall, Jr. et al.) teach that a well can be insulated by boiling a sodium silicate solution in contact with a well tubing to form a thin coating of silicate foam on the outside of the tubing string. However, to improve thermal insulation efficiency, excess silicate solution must be water-, gas- or steam lifted from the annulus. If excess sodium silicate solution is not removed from the wellbore, it convects locally and greatly lowers the thermal insulation efficiency of the sodium silicate foam. Although it may be saved for future use, handling of hot alkali solution is extremely dangerous because of its high temperature, high pH and reactivity.
Any advantages of the methods of the prior art are very difficult to obtain when very small tubing and casing is used, for example 3.175 cm (1.250 inch) injection tubing in 6.032 cm (2.375 inch) casing. When such small tubing and casing is used, the injection tubing should be permanently latched into the casing prior to introducing steam. Also, the injection tubing size is too small to install fixtures, such as gas lift valves. Therefore, no mechanism currently remains available to pump out the excess sodium silicate fluid out of the annulus.